Immunoglobulin G (IgG) – Definition & Function
Immunoglobulin G (IgG) is the most abundant antibody type in human blood and a key component of the immune system for fighting infections.
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Immunoglobulin G (IgG) is the most abundant antibody type in human blood and a key component of the immune system for fighting infections.
What is Immunoglobulin G?
Immunoglobulin G (IgG) is the most prevalent class of antibody found in human blood, accounting for approximately 70–80% of all circulating antibodies. It is a glycoprotein produced by plasma cells, which are specialized immune cells derived from B lymphocytes. IgG plays a central role in defending the body against bacterial and viral infections and in regulating immune responses.
Structure and Properties
IgG consists of two heavy chains and two light chains connected by disulfide bonds, forming a characteristic Y-shaped structure. It is divided into four subclasses: IgG1, IgG2, IgG3, and IgG4, each with slightly different abundances and biological functions. Notably, IgG is the only antibody class capable of crossing the placenta, thereby providing newborns with passive immunity during the first months of life.
Functions in the Immune System
- Neutralization of pathogens: IgG binds to viruses and bacteria, preventing them from entering and infecting host cells.
- Opsonization: IgG coats pathogens, marking them for recognition and destruction by phagocytic immune cells.
- Complement activation: IgG can trigger the complement system, a cascade of proteins that assists in eliminating pathogens.
- Antibody-dependent cellular cytotoxicity (ADCC): IgG enables natural killer cells to destroy infected or malignant cells.
- Immunological memory: Following infection or vaccination, specific IgG antibodies persist in the bloodstream, providing rapid protection upon re-exposure to the same pathogen.
Clinical Significance
IgG Deficiency
A deficiency in IgG, known as hypogammaglobulinemia, results in increased susceptibility to recurrent bacterial infections, particularly of the respiratory tract. Causes may be congenital, such as Common Variable Immunodeficiency (CVID), or acquired through medications or conditions such as multiple myeloma. Treatment typically involves regular immunoglobulin replacement therapy.
Elevated IgG Levels
Elevated IgG concentrations can indicate chronic infections, autoimmune diseases (e.g., systemic lupus erythematosus, rheumatoid arthritis), liver disease, or plasma cell disorders. Temporary increases in IgG are also expected following vaccination or recovery from infection.
IgG as a Diagnostic Tool
Measuring specific IgG antibodies in the blood is used to diagnose past infections (e.g., Lyme disease, toxoplasmosis, COVID-19) and to assess vaccine-induced immunity. In allergy diagnostics, IgG4 levels are monitored to evaluate tolerance development during allergen immunotherapy.
IgG as a Therapeutic Agent
Intravenous or subcutaneous immunoglobulin preparations (IVIG/SCIG), derived from pooled human plasma, are used to treat primary immunodeficiencies, autoimmune conditions, and inflammatory neuropathies. Clinical applications include the treatment of Guillain-Barre syndrome, immune thrombocytopenia (ITP), and chronic inflammatory demyelinating polyneuropathy (CIDP).
References
- Janeway CA Jr et al. - Immunobiology: The Immune System in Health and Disease. 9th edition. Garland Science, 2017.
- World Health Organization (WHO) - Guidelines on the quality, safety and efficacy of biotherapeutic protein products prepared by recombinant DNA technology. WHO Technical Report Series, 2013.
- Bonilla FA, Barlan I, Chapel H et al. - International Consensus Document (ICON): Common Variable Immunodeficiency Disorders. Journal of Allergy and Clinical Immunology: In Practice, 2016.
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